Lawrence Livermore National Laboratory (LLNL), operated by Lawrence Livermore National Security (LLNS) under contract with the U.S. Department of Energy (DOE), is offering the opportunity for partnership and licensing of a new technology for the storage of electrical energy in modular “electromechanical batteries” (EMB) designed for land-based vehicular applications. The technology embodies several novel concepts for storing electrical energy as rotational energy. The new design concepts outlined in this document specifically address the requirements of vehicular applications, e.g. those of electric-powered automobiles, both full electrics and hybrids. The objectives of the proposed new development include designing and constructing modular EMBs having a degradation-free and long service life with minimal maintenance, low rate of self-discharge, high electrical turnaround efficiency (important for optimizing regenerative braking), and the ability to withstand vehicular “g” forces. If the development is successful, the new EMBs are also predicted by computer-based simulations to deliver a substantial increase in available peak power and a doubling of the driving range as compared to electrochemical battery packs of comparable size and weight.

Description

The Laboratory has several decades of experience in the development of EMBs. In the course of this development several prototypes that advanced the technology were constructed and their designs were licensed. Work is now underway at the Laboratory to develop new types of EMBs with the objective of bulk storage of electricity at utility scale. This new generation of modular flywheel storage systems is based on the use of some special technologies, including passive magnetic bearings and a novel type of electrostatic generator that is very light in weight and, since it has near-100 percent efficiency, does not require active cooling. In these respects these new designs differ greatly from present commercially available flywheel storage systems. As noted above, the purpose of this FBO is to make U. S. Industry aware of an opportunity to participate in the development of EMBs specifically designed for vehicular use.

With minor exceptions, at present all electric vehicles are powered by electrochemical batteries, with their well-known limited deep-discharge cycle life and modest electrical turnaround efficiency (70 to 80 percent). Electrochemical batteries in principle could continue to fill the storage demands of electric vehicles, but they fall short in three areas. The first is their limited cycle life under deep discharge conditions. Despite many decades of development the best of such batteries typically have a deep-discharge cycle life of about 1000 cycles, requiring operation under restricted depths of discharge to achieve multi-year lifetimes. Second, the “turnaround” energy efficiency of electrochemical batteries, being typically of order 80 percent, lowers the overall energy efficiency of the vehicle. It also limits the efficiency of regenerative braking, a technique that is important in increasing the range of the vehicle per kWh in its battery pack. Third, the electrochemical batteries in present use carries with them problems of overheating and catching on fire, and also involve the disposal of hazardous waste, problems that are not an issue with the EMB.

(special permanent magnet arrays are used to levitate and to dynamically stabilize the rotating flywheel system, eliminating the need for complicated sensors and control circuits, which in turn, eliminates heat generation in the vacuum chamber and the corresponding active cooling system required).

Long device lifetime as a consequence of 100% passive magnetic bearing design (no electromagnets), resulting in no frictional wearing of components and no heat-related issues.

No loss of capacity from number of charge-discharge cycles.

Near-zero internal heat losses from the electrostatic generator/motor and the passive magnetic bearing system, thus no requirement for an active cooling system.

Modularity, The Battery Pack, and Predicted Range:

As is the case with electrochemical batteries when they are used in electric vehicles, an EMB battery pack would be made up of many individual “cells”. That is, the pack would be made up of several EMBs whose rotors would likely be about the size of a large coffee can. To minimize external gyroscopic effects the battery pack would be made up of pairs of counter-rotating flywheels. The pack would then contain as many such pairs as required to achieve the desired range of the vehicle. Preliminary computer-based studies of example designs indicate that for a comparable weight and size of battery pack, the EMB-based one should be capable of doubling the range of the vehicle as compared to today’s electric vehicles based on present-day electrochemical batteries.

Description of New Technologies:

The design calculations that have been performed in exploring the potentialities of LLNL’s new approaches to flywheel energy storage have been built on existing and past LLNL flywheel programs, including a program aimed at flywheel systems for the bulk storage of electricity at utility scale. To achieve the requirements of such systems, as mentioned above, LLNL has developed some key new technologies, technologies that we believe are unique to flywheel energy storage. These developments came about because the LLNL researchers came to the conclusion that with vehicular applications, as with the present program on bulk storage, the proposed new-generation flywheels must break with past tradition in critical technological areas.

The first of these is the generator/motor. In all commercially available flywheel storage systems built to date, this component consists of an electromagnetic-type generator. This generator must operate within the evacuated housing of the flywheel rotor, using internally mounted high-field permanent magnets and internally located windings. In addition to the mechanical issue of the high centrifugal force load caused by the magnets, even when no power is drawn from the system there exist parasitic losses associated with eddy currents induced in the windings and in other metallic parts. When power is drawn, the ohmic power losses in the windings lower the system efficiency and represent a substantial heat source that requires an active cooling system.

The LLNL-proposed answer to the first of the three technological areas is to replace the electromagnetic generator with an electrostatic (E-S) one, building on the pioneering work of John Trump at M.I.T. in the 1950’s. In this type of generator/motor the heavy magnets located on the inner surface of the flywheel rotor are replaced by lightweight electrodes (the generator/motor “rotor”). These electrodes face another set of electrodes (the “stator”) that, together with the rotor electrodes, form a time-varying capacitor. The system is charged through special charging circuits that employ parametric resonance effects that can increase the power output by more than an order of magnitude over that obtainable using the Trump design. In this type of generator the internal efficiency of the generator is essentially 100 percent (the only significant losses are in the external power electronic components that permit generator (discharge) and/or motor (charge) functions. When not generating or motoring, the parasitic losses of this system are zero. Also, preliminary experimental model tests together with computer simulations have shown that the generator output of this new E-S generator/motor should be more than adequate for vehicular applications, actually substantially exceeding the power output of present electric vehicle battery packs.

The fact that the peak power output of the electrostatic generators of an EMB battery pack for an electric vehicle can substantially exceed the peak power requirement of the vehicle means that the electrostatic generators of the battery pack can deliver the power required for the vehicle through their parametric resonance circuits to the output electrical bus at a very high efficiency. In an example case where the EMB battery pack power output was assumed to be operated at 50 percent of its absolute peak value the calculated generator efficiency at the output bus of each module of the pack was 98.5 percent, far higher than that possible from an electrochemical battery pack.

The second key technological innovation is the bearing system. Because mechanical bearings operating in a vacuum have limited lifetimes and significant parasitic losses, present-day commercial flywheel storage systems have adopted the use of magnetic bearings to support the spinning rotor. However, all magnetic levitation systems, including maglev trains, must deal with the constraints imposed by Earnshaw’s theorem. This theorem, posited in the early 1800’s, shows that it is impossible to stably levitate any array of permanent magnets by the magnetic forces exerted by another, stationary, array of permanent magnets. A loophole for the magnetic bearing problem is that Earnshaw’s Theorem only applies to static systems. Stable levitation is possible if dynamic effects can be employed. The commercial flywheel systems therefore employ what is known as “active” magnetic bearings. In an active bearing system, electromagnets driven by power circuits that are controlled by sensors that detect unstable motion and, in turn, control the magnet power. Such bearing systems are expensive, require maintenance, and represent an internal parasitic power loss (in the electromagnets) that requires active cooling, and can lead to appreciable energy losses during standby times of the flywheel system.

Given the above-listed negative features of active magnetic bearing systems the LLNL approach is to replace the stabilizing elements of the active magnetic bearing by a purely “passive” stabilizer. That is to say the entire magnetic bearing system is composed simply of annular arrays of permanent magnets that provide contactless levitation. Their Earnshaw-based instability is overcome by a passive system that uses flywheel rotation to generate a stabilizing force. An example of such a stabilizer is one that consists of a set of magnets on the inner surface of the rotor (typically using “Halbach arrays” of permanent-magnet bars). These rotating arrays interact with a stationary set of windings to produce a restoring force that overcomes the inherent instability of the levitating permanent magnets. In a typical form of the stabilizer the windings are located at a “null” point in the Halbach array magnetic fields so that current is only induced in the stabilizer windings upon motion away from the force equilibrium plane of the levitating magnets. In operation, therefore, the losses of the stabilizer approach zero, except in those cases when there exist accelerations of external origin.

Since in vehicular applications the EMBs will experience g-level accelerations in normal driving, momentary-contact “touchdown” bearing concepts have been conceived and these would be investigated in the course of the development.

Predicted Self-Discharge Times:

For electric vehicular use an important parameter of the energy storage system is its self-discharge time. For the new LLNL EMB we believe that this time can be many weeks in duration, for the following reasons: First, standby losses of the E-S generator are zero. Second, when the vehicle is parked the losses of the passive bearing system can be reduced to near zero. Contrast this with flywheel systems with integrated permanent-magnet-based generator/motors and active magnetic bearings. Here there are eddy-current-based losses and active-bearing power demands that substantially shorten the self-discharge times.

The remaining source of losses is aerodynamic friction. At the operating vacuums, of order 10-5 Torr, calculated self-discharge times from aerodynamic friction are several months. That such vacuum pressures can be maintained in a sealed-off vacuum vessel containing a carbon-fiber/epoxy rotor was demonstrated in a flywheel-related experiment performed at LLNL in the 1990s. In this experiment, after the vacuum vessel was pumped down long enough to outgas the rotor, it was sealed off and its pressure was monitored. It was found that the pressure stabilized at 2.0 x 10-5 Torr, remaining constant for a period of two months, at which time the experiment was accidently terminated.

Energy and Power Density Parameters:

Using computer simulation codes that were benchmarked either by independent code calculations or by laboratory test models, the critical parameters of energy density (watt-hours/kg) and power density (kw/kg) were calculated for modular EMBs that were of a size that would be appropriate to package together to form battery packs with dimensions comparable to those of present-day electric automobiles, such as the Tesla Model S or the Nissan Leaf. One example calculation employed EMBs each of which occupied a cubical volume measuring 30 cm. on a side, i.e. a volume of 0.027 cubic meters. The volume of the Leaf battery pack is approximately 0.5 cubic meters so that one could fit 18 of the example EMBs into a battery pack of the same volume.

Assuming a rotor employing high-strength T1000 carbon fiber and epoxy composite, each EMB in the example case would store about 2.5 kWh of energy (90 percent of the full-charge energy, assuming discharge to 30 percent of initial rpm) so that the total stored energy would be about 45 kWh, more than twice that of the present Leaf battery pack. Also, the calculated peak power output at full charge of the special-design electrostatic generator of a single EMB was 50 kW. Thus in principle the present 90 kW requirement could be met by only two of the EMB battery pack modules. In practice this would allow the operation of the battery pack EMBs at power levels well below their full-charge peak power.

Comparing the calculated Watt-hours per kilogram of the example EMB battery pack with that of the Leaf battery pack, the latter is 68 Wh/kg, while the estimated EMB battery pack value using T1000 carbon fiber composite is 200 Wh/kg. (Using the lower-cost IMS65 carbon fiber the code-predicted energy density is 140 Wh/kg.)

Module Size Considerations:

In the discussion above an example was given of a LEAF-size battery pack made of 18 EMBs each of which would occupy a cubical volume measuring 30 cm. on a side. An optimized design, however, might use EMBs of smaller dimensions, owing to the scaling laws of important performance parameters that apply. For example, since the power output of the E-S generator scales as the square of the rotor radius, while the energy stored varies as the cube of the radius, smaller radii are favored if high power density (Watts/m3) is desired. Also, and especially relevant for vehicular applications, is the fact that the ratio of angular momentum of the rotor to stored energy decreases with rotor radius, again favoring smaller rotors. Thus the optimum size of the EMB rotors might be substantially smaller than the one picked for the example, as determined finally by unit cost and system complexity issues.

Special Issues for Vehicular Use of EMBs:

To complete the description of the computational and design results to date, they have also included conceptual designs of all of the major components of an automotive EMB module, including “touchdown bearings” to handle the expected accelerations that would occur in operating the vehicle, together with conceptual designs of the vacuum vessel that would also act to contain the fragments resulting from failure of the rotor together with the fragments of other elements, such as the magnetic bearings and the electrostatic generator. As was shown in a picture that was obtained of a failed commercial flywheel with a carbon-fiber composite rotor, upon failure the rotor turned into “cotton candy” that was completely contained by the vacuum chamber. Only if there are heavy and structurally strong rotor components, such as large rare earth magnets for the generator/motor, does containment become a serious issue. Because of the use of an electrostatic generator motor with its lightweight rotor electrodes, together with passive bearings made up of small magnet elements, the new-generation LLNL EMBs should have a minimal containment problem.

Current Status of Development and Go Forward Plan:

The development and testing of LLNL’s EMB system is a “work in progress” at this time. The present effort is directed toward the bulk storage of electricity for utility and other applications. Both an E-S generator/motor system and a passive magnetic bearing system have been designed and fabricated but are in various stages of assembly and have not been through our test validation program. The design of these systems is based on the predictions of several computer codes that were written to evaluate the theoretical design model and system performance. To further substantiate our design philosophy, an independent code evaluation was done by a professor and his graduate student at Carnegie-Mellon University. Their code results confirmed those obtained at LLNL.

Consequently a key result from this phase of work is to show that the measured performance/empirical data obtained so far agree closely with the code predictions. Benchmarking the computer codes developed to predict system performance is a key milestone in demonstrating the EMB’s viability as an energy storage system.

The work to-date directed toward EMBs for vehicular applications has been limited to computer simulations using the above-described codes and to preliminary designs. These calculations have been very encouraging, but clearly need to be confirmed experimentally using real hardware. Continuation of this work would therefore involve the design and testing of prototypes en route to a complete unit. At that point the development and testing would be ready to be taken over by Industry, en route to commercial production.

A notional schedule of our Go-Forward plan is shown depicting the major activities on an LLNL webpage. We are very conscious of the desire to have a working prototype as soon as possible, and the proposed work plan supports that understanding. The first 9 months of the program will be to engineer, design and build a working prototype, typical in size to what would actually be used in a vehicle. The following 3 months would be dedicated to testing this unit in our High Pressure Laboratory at LLNL, which has been prepared and approved to support this test program. Testing will include understanding the flywheels energy storage capabilities, system efficiency, etc. and comparing the real data with our computer simulation codes and predictions. A preliminary budgetary cost of $10M for the Year 1 effort is proposed.

If additional funding became available, the first six months of Year 2 could be used for in-vehicle testing, gathering real on-the-road test data, and ensuring that the engineering challenges of vehicle shock, vibration, maneuvering (cornering, braking and accelerating) and gyro-dynamics have been successfully addressed. The in-vehicle tests could be accomplished, for example, by replacing the 1.2 kWh battery pack of a Prius hybrid auto by one or two of the prototype EMBs developed in the Year 1 program. In an earlier, similar, in-vehicle testing program at LLNL, a Prius was converted to operate on hydrogen and then was equipped with a composite-based high-pressure hydrogen fuel tank that had been developed at the Lab. It was then driven around the Laboratory for a distance of 650 miles without refueling.

Separators for flywheel rotorsA separator forms a connection between the rotors of a concentric rotor assembly. This separator allows for the relatively free expansion of outer rotors away from inner rotors while providing a connection between the rotors that is strong enough to prevent disassembly. The rotor assembly includes at least two rotors referred to as inner and outer flywheel rings or rotors. This combination of inner flywheel ring, separator, and outer flywheel ring may be nested to include an arbitrary number of concentric rings. The separator may be a segmented or continuous ring that abuts the ends of the inner rotor and the inner bore of the outer rotor. It is supported against centrifugal loads by the outer rotor and is affixed to the outer rotor. The separator is allowed to slide with respect to the inner rotor. It is made of a material that has a modulus of elasticity that is lower than that of the rotors.

Electrostatic generator/motor configurationsElectrostatic generators/motors designs are provided that include a stator fixedly connected to a first central support centered about a central axis. The stator elements are attached to the first central support. Similarly, a second stator is connected to a central support centered about the central axis, and the second stator has stator elements attached to the second central support. A rotor is located between the first stator and the second stator and includes an outer support, where the rotor is rotatably centered about the central axis, the rotor having elements in contact with the outer support, each rotor element having an extending rotor portion that extends radially from the outer support toward the axis of rotation.

Concentric ring flywheel without expansion separatorsA concentric ring flywheel wherein the adjacent rings are configured to eliminate the need for differential expansion separators between the adjacent rings. This is accomplished by forming a circumferential step on an outer surface of an inner concentric ring and forming a matching circumferential step on the inner surface of an adjacent outer concentric ring. During operation the circumferential steps allow the rings to differentially expand due to the difference in the radius of the rings without the formation of gaps therebetween, thereby eliminating the need for expansion separators to take up the gaps formed by differential expansion.

Lightweight flywheel containmentA lightweight flywheel containment composed of a combination of layers of various material which absorb the energy of a flywheel structural failure. The various layers of material act as a vacuum barrier, momentum spreader, energy absorber, and reaction plate. The flywheel containment structure has been experimentally demonstrated to contain carbon fiber fragments with a velocity of 1,000 m/s and has an aerial density of less than 6.5 g/square centimeters. The flywheel containment, may for example, be composed of an inner high toughness structural layer, and energy absorbing layer, and an outer support layer. Optionally, a layer of impedance matching material may be utilized intermediate the flywheel rotor and the inner high toughness layer.

Passive magnetic bearing element with minimal power lossesSystems employing passive magnetic bearing elements having minimal power losses are provided. Improved stabilizing elements are shown, employing periodic magnet arrays and inductively loaded circuits, but with improved characteristics compared to the elements disclosed in U.S. Patent No. 5,495,221 entitled "Dynamically Stable Magnetic Suspension/Bearing System." The improvements relate to increasing the magnitude of the force derivative, while at the same time reducing the power dissipated during the normal operation of the bearing system, to provide a passive bearing system that has virtually no losses under equilibrium conditions, that is, when the supported system is not subject to any accelerations except those of gravity.

Dynamically stable magnetic suspension/bearing systemA magnetic bearing system contains magnetic subsystems which act together to support a rotating element in a state of dynamic equilibrium. However, owing to the limitations imposed by Earnshaw's Theorem, the magnetic bearing systems to be described do not possess a stable equilibrium at zero rotational speed. Therefore, mechanical stabilizers are provided, in each case, to hold the suspended system in equilibrium until its speed has exceeded a low critical speed where dynamic effects take over, permitting the achievement of a stable equilibrium for the rotating object. A state of stable equilibrium is achieved above a critical speed by use of a collection of passive elements using permanent magnets to provide their magnetomotive excitation. The magnetic forces exerted by these elements, when taken together, levitate the rotating object in equilibrium against external forces, such as the force of gravity or forces arising from accelerations. At the same time, this equilibrium is made stable against displacements of the rotating object from its equilibrium position by using combinations of elements that possess force derivatives of such magnitudes and signs that they can satisfy the conditions required for a rotating body to be stably supported by a magnetic bearing system over a finite range of those displacements.

Electrostatic generator/motor having rotors of varying thickness and a central stator electrically connected together into two groupsA sub-module consists of a set of two outer sets of stationary fan-blade-shaped sectors. These outer sectors include conductive material and are maintained at ground potential in several examples. Located midway between them is a set of stationary sector plates with each plate being electrically insulated from the others. An example provides that the inner sector plates are connected together alternately, forming two groups of parallel-connected condensers that are then separately connected, through high charging circuit resistances, to a source of DC potential with respect to ground, with an additional connecting lead being provided for each group to connect their output as an AC output to a load. These same leads can he used, when connected to a driver circuit, to produce motor action.

Interlayer toughening of fiber composite flywheel rotorsAn interlayer toughening mechanism to mitigate the growth of damage in fiber composite flywheel rotors for long application. The interlayer toughening mechanism may comprise one or more tough layers composed of high-elongation fibers, high-strength fibers arranged in a woven pattern at a range from 0.degree. to 90.degree. to the rotor axis and bound by a ductile matrix material which adheres to and is compatible with the materials used for the bulk of the rotor. The number and spacing of the tough interlayers is a function of the design requirements and expected lifetime of the rotor. The mechanism has particular application in uninterruptable power supplies, electrical power grid reservoirs, and compulsators for electric guns, as well as electromechanical batteries for vehicles.

Combined passive magnetic bearing element and vibration damperA magnetic bearing system contains magnetic subsystems which act together to support a rotating element in a state of dynamic equilibrium and dampen transversely directed vibrations. Mechanical stabilizers are provided to hold the suspended system in equilibrium until its speed has exceeded a low critical speed where dynamic effects take over, permitting the achievement of a stable equilibrium for the rotating object. A state of stable equilibrium is achieved above a critical speed by use of a collection of passive elements using permanent magnets to provide their magnetomotive excitation. In a improvement over U.S. Pat. No. 5,495,221, a magnetic bearing element is combined with a vibration damping element to provide a single upper stationary dual-function element. The magnetic forces exerted by such an element, enhances levitation of the rotating object in equilibrium against external forces, such as the force of gravity or forces arising from accelerations, and suppresses the effects of unbalance or inhibits the onset of whirl-type rotor-dynamic instabilities. Concurrently, this equilibrium is made stable against displacement-dependent drag forces of the rotating object from its equilibrium position.

Concentric ring flywheel with hooked ring carbon fiber separator/torque couplerA concentric ring flywheel with expandable separators, which function as torque couplers, between the rings to take up the gap formed between adjacent rings due to differential expansion between different radius rings during rotation of the flywheel. The expandable separators or torque couplers include a hook-like section at an upper end which is positioned over an inner ring and a shelf-like or flange section at a lower end onto which the next adjacent outer ring is positioned. As the concentric rings are rotated the gap formed by the differential expansion there between is partially taken up by the expandable separators or torque couplers to maintain torque and centering attachment of the concentric rings.

Electrostatic generator/motor configurationsElectrostatic generators/motors designs are provided that generally may include a first cylindrical stator centered about a longitudinal axis; a second cylindrical stator centered about the axis, a first cylindrical rotor centered about the axis and located between the first cylindrical stator and the second cylindrical stator. The first cylindrical stator, the second cylindrical stator and the first cylindrical rotor may be concentrically aligned. A magnetic field having field lines about parallel with the longitudinal axis is provided.

Centrifugally decoupling touchdown bearingsCentrifugally decoupling mechanical bearing systems provide thin tensioned metallic ribbons contained in a support structure. This assembly rotates around a stationary shaft being centered at low speeds by the action of the metal ribbons. Tension springs are connected on one end to the ribbons and on the other end to the support structure. The ribbons pass through slots in the inner ring of the support structure. The spring preloading thus insures contact (or near-contact) between the ribbons and the shaft at rotation speeds below the transition speed. Above this speed, however, the centrifugal force on the ribbons produces a tensile force on them that exceeds the spring tensile force so that the ribbons curve outward, effectively decoupling them from mechanical contact with the shaft. They still remain, however, in position to act as a touchdown bearing in case of abnormally high transverse accelerations.